Background of the Invention and Introduction
In hot natural environments, humans normally excel at maintaining correct
body temperature. Blood flow through the skin increases and perspiration
provides cooling through evaporation. However, when protective outer
clothing must be worn, the evaporation of perspiration is hindered rendering
the body's natural cooling mechanisms ineffective with consequent increase
in body temperature. If correct work-rest regiments are not followed,
individuals can suffer impaired concentration, fatigue and loss of human
efficiency.
When the body's natural cooling mechanism are ineffective, cool liquid/air
flows through liquid/air circulating helmet, replacing natural cooling. In hot
environments these helmet also intercept heat from external sources,
creating a cool 'microclimate' to isolate the head from the heat.
The hybrid thermoelectrically cooled/heated back-packed air circulating
helmet system according to the present invention provides powerful
solutions to the problems presented by demanding thermal conditions in
industrial environments such as a steel plant.
Principle
The solid state cooling microclimate-conditioning unit according to the
present invention is basically a cooling/heating system based on solid state
thermoelectric. Thermoelectric Cooling/Heating is based on the Peltier effect
in which when a current is passed around a circuit of a different materials,
one junction gets heated while the other junction is cooled.

By reversing the direction of current flow, the heating and cooling of the two
junctions are mutually interchanged.
Heat Transfer Process:
Heat transfer is the transfer of heat from one place to another by movement
of fluids/air. When an object is at a different temperature from its
surroundings or another object, heat exchange occurs in such a way that
the body and the surroundings reach thermal equilibrium. This means that
they are at the same temperature. Heat transfer always occurs from a
higher-temperature object to a cooler-temperature. Where there is a
temperature difference between objects in proximity, heat transfer between
them can never be stopped; it can only be decreased.
There are mainly three methods for the heat transfer:
* Conduction
* Convection
* Radiation
Conduction:
Conduction is the process of heat transfer through a medium or material
without any movement of the medium or material. Conduction is the
transfer of heat by direct contact of particles of matter. The transfer of
energy could be primarily by elastic impact as in fluids or by free electron
diffusion as predominant in metals or phonon vibration as predominant in
insulators. A heat pipe is a passive device that is constructed in such a way
that it acts as though it has extremely high thermal conductivity.

a) Steady-state conduction: Steady state conduction is the form of
conduction which happens when the temperature difference driving
the conduction is constant so that after an equilibration time, the
spatial distribution of temperatures (temperature field) in the
conducting object does not change any further. In steady state
conduction, the amount of heat entering a section is equal to amount
of heat coming out. In steady state conduction, all the laws of direct
current electrical conduction can be applied to "heat currents". In
such cases, it is possible to take "thermal resistances" as the analog
to electrical resistances. Temperature plays the role of voltage and
heat transferred is the analog of electrical current.
b) Transient conduction: There also exists non-steady-state situations
in which the temperature drop or increase occurs more drastically,
such as when a hot copper ball is dropped into oil at a low
temperature. Here the temperature field within the object changes as
a function of time, and the interest lies in analyzing this spatial change
of temperature within the object over time. This mode of heat
conduction can be referred to as transient conduction. Analysis of
these systems is more complex and (except for simple shapes) calls
for the application of approximation theories, and/or numerical
analysis by computer.
Convection:
Convection is the transfer of heat from one place to another by the
movement of fluids. The presence of bulk motion of the fluid enhances the
heat transfer between the solid surface and the fluid.

There are two types of convective heat transfer:
• Natural convection: when the fluid motion is caused by buoyancy
forces that result from the density variations due to variations of
temperature in the fluid. For example, in the absence of an external
source, when the mass of the fluid is in contact with a hot surface, its
molecules separate and scatter, causing the mass of fluid to become
less dense. When this happens, the fluid is displaced vertically or
horizontally while the cooler fluid gets denser and the fluid sinks. Thus
the hotter volume transfers heat towards the cooler volume of that
fluid.
• Forced convection: when the fluid is forced to flow over the surface by
external source such as fans and pumps, creating an artificially induced
convection current.
Internal and external flow can also classify convection. Internal flow occurs
when the fluid is enclosed by a solid boundary such as a flow through a
pipe. An external flow occurs when the fluid extends indefinitely without
encountering a solid surface. Both of these convections, either natural or
forced, can be internal or external because they are independent of each
other.
Radiation:
Radiation is the transfer of heat energy through empty space. All objects
with a temperature above absolute zero radiate energy at a rate equal to
their emissivity multiplied by the rate at which energy would radiate from
them if they were a black body.

Summary of the Invention
The present invention provides a thermoelectrically cooled/heated back-pack
helmet system comprising a hybrid heat sink unit with thermoelectric
module, small fans, blower, rechargeable battery with charger to be carried
as back pack and an electrical switch fixed with waist belt, wherein said
hybrid heat sink unit is both convective and heat pipe; and wherein said
thermoelectric module consists of two or more elements of n- and p-type
doped semiconductor materials connected electrically in series and thermally
in parallel; thermoelectric elements and electrical interconnects being
typically mounted between two ceramic substrates for mechanical stability
and electrical insulation from one another and from external mounting
surfaces; and the heat actively pumped through the thermoelectric module
(cooling capacity) being proportional to the magnitude of the applied DC
electric current and thermal conditions on each side of the module.
Cold air is carried by the said blower through a flexible pipe to the head of
the user, the cold air being distributed for cool the head. The flexible pipe
enters through holes in the helmet and through the small holes in the pipe.
The hybrid heat sink unit is both convective and heat pipe. The
thermoelectric module consists of two or more elements of n and p-type
doped semiconductor materials connected electrically in series and thermally
in parallel. The thermoelectric elements and their electrical interconnects are
typically mounted between two ceramic substrates for mechanical stability
and electrical insulation from one another and from external mounting
surfaces. In a preferred embodiment, the TE modules range in size from
approximately 2.5-50 mm (0.1 to 2.0 inches) square and 2.5-5 mm (0.1 to
0.2 inches) in height. The heat actively pumped through the TE module
(cooling capacity) is proportional to the magnitude of the applied DC electric

current and thermal conditions on each side of the module. The heat flow
and surface temperature can be regulated by varying the input current. In a
preferred embodiment, a heat spreader comprising a copper plate having
high thermal conductivity may be used. The thermoelectric module (TEC) is
divided into two parts namely cold side and hot side. The cold side
components comprise blower, ducts and cold plate. The said ducts may be
convergent or divergent. The hot side components comprise aluminium
block, heat pipe, fins and fan. Optionally, there is provided a heat sink
assembly comprising components like inlet duct (divergent), cold plate,
outlet duct (convergent), TEC modules, aluminium block, heat pipe, fins,
and fan. The TEC module takes heat from hot air through the cold side and
dissipates it through hot side. The TEC modules are used in parallel on one
side of the cold plate and the heat dissipated is transferred to heat pipe
through aluminium block. Equally spaced hot side fins may be used to
dissipate heat to the atmosphere. A hot side fan is preferably used to
facilitate forced convection heat transfer from hot side fins.
The present invention also provides a method for predicting the
performance of a back-pack helmet system using computational fluid
dynamics (CFD) analysis, said method involving the steps of: choosing
volume flow rate value for the blower matching cold plate pressure drop
from given blower characteristics; performing similar procedure for hot side
fan; calculating corresponding cold plate outlet temperature matching TEC
module input power requirements; predicting mean baffle temperature;
calculating corresponding heat transfer coefficient value; using the
calculated heat transfer coefficient for predicting new coefficient for different
values of blower and hot side fan volume flow rate; and calculating values of
temperature at the hot side of TEC module and total heat dissipated from
hot side.

Brief Description of the Accompanying Drawings
Figure 1 shows overall assembly of hybrid system.
Figure 2 shows a blower used in a preferred embodiment of the invention.
Figure 3 shows a fan used in the analysis of volume flow rate for a fan being
investigated for desired design performance.
Figure 4 shows dimensions of initial cold plate and domains for
computational fluid dynamics (CFD) analysis.
Figure 5 shows multi-block topology used for the CFD simulation.
Figures 6 and 7 show corresponding "fluid" and "solid" zones in generated
volume mesh respectively.
Figure 8 shows boundary conditions applied to the CFD domain in order to
carry out CFD simulations.
Figures 9 and 10 show pressure and velocity-X variations computed through
CFD for the domain.
Figure 11 shows temperature profile (in °K) on the section taken at the
middle of domain when viewed from front.
Figure 12 shows the temperature contours when section is taken along
normal plane when viewed from top.
Figure 13 shows initial design - volume flow rate selection for blower.

Figure 14 shows variation of cold plate exit temperature with hot side fan
with different TEC modules.
Figure 15 shown initial design - volume flow rate selection for blower.
Figure 16 shows variation of cold plate exit temperature with hot side fan
with different TEC modules.
Figure 17 shows design of alternate cold plate.
Figure 18 shows CFD domain volume mesh generated for alternate design.
Figure 19 shows boundary conditions applied to the CFD domain in order to
carry out the simulations for alternate design.
Figures 20 and 21 show pressure and velocity-X variations computed
through CFD for the domain for alternate design.
Figure 22 shows temperature profile on the section taken at the middle of
domain when viewed from front for alternate design.
Figure 23 shows the temperature contours when section is taken along
normal plane when viewed from side for alternate design.
Figure 24 shows alternate design - volume flow rate selection for blower.
Figure 25 shows alternate design - Exit Temperature vs Fan Volume Flow
Rate.

Figure 26 shows alternate design - volume flow rate selection for blower.
Figure 27 shows alternate design - Exit Temperature vs Fan Volume Flow
Rate.
Figure 27A shows effect of blower Volume Flow Rate on Cold Plate Exit
Temperature.
Figure 28 shows the cooling and comfort level assessment in R&D
Laboratory.
Figure 29 shows the front view of helmet modified for application.
Figure 30 shows the casing outline of system.
Figure 31 shows the casing outline along with blower & cold plate.
Figures 32 and 33 show casing outline along with heat pipe.
Figures 34 and 35 show casing outline along with blower and heat pipe.
Figures 36 and 37 show testing of back-pack helmet in hot chamber at R&D
Laboratory.
Figure 38 shows outline of battery pack (nominal voltage 11.1) external
dimension in soft casing.
Figures. 39 and 40 show battery configuration of cooling / heating helmet
assembly.

Figure 41 shows outline of battery pack (nominal voltage 14.8) external
dimension in soft casing.
Figure 42 shows outline of battery pack (nominal voltage 14.8) external
dimension in hard casing.
Figures 43 and 44 show architectural engineering drawing of assembly.
Figure 45 shows the architectural drawing of full assembly.
Figure 46 schematic drawing of user wearing cooling / heating system.
Detailed Description of the Invention
Thermoelectric Cooling:
Thermoelectric cooling uses the Peltier effect to create a heat flux between
the junctions of two different types of materials. A Peltier cooler, heater, or
thermoelectric heat pump is a solid-state active heat pump which transfers
heat from one side of the device to the other side against the temperature
gradient (from cold to hot), with consumption of electrical energy. Such an
instrument is also called a Peltier device, Peltier heat pump, solid state
refrigerator, or thermoelectric module.
Since heating can be achieved more easily and economically by many other
methods, Peltier devices are mostly used for cooling. However, when a
single device is to be used for both heating and cooling, a Peltier device may
be desirable. Simply connecting it to a DC voltage will cause one side to
cool, while the other side warms. The effectiveness of the pump at moving

the heat away from the cold side is dependent upon the amount of current
provided and how well the heat can be removed from the hot side.
By applying a low voltage DC power to a TE module, heat will be moved
through the module from one side to the other. One module face, therefore,
will be cooled while the opposite face is simultaneously heated. It is
important to note that this phenomenon may be reversed whereby a change
in the polarity (plus and minus) of the applied DC voltage will cause heat to
be moved in the opposite direction. Consequently, a thermoelectric module
may be used for both heating and cooling thereby making it highly suitable
for precise temperature control applications. A thermoelectric module can
also be used for power generation. In this mode, a temperature differential
applied across the module will generate a current.
A practical thermoelectric module generally consists of two or more
elements of n and p-type doped semiconductor materials that are connected
electrically in series and thermally in parallel. These thermoelectric elements
and their electrical interconnects typically are mounted between two ceramic
substrates. The substrates hold the overall structure together mechanically
and electrically insulate the individual elements from one another and from
external mounting surfaces. Most thermoelectric modules range in size from
approximately 2.5-50 mm (0.1 to 2.0 inches) square and 2.5-5 mm (0.1 to
0.2 inches) in height. A variety of different shapes, substrate materials,
metallization patterns and mounting options are available.
Cooling capacity (heat actively pumped through the thermoelectric module)
is proportional to the magnitude of the applied DC electric current and the
thermal conditions on each side of the module. By varying the input current
from zero to maximum, it is possible to regulate the heat flow and control
the surface temperature.

Thermoelectric modules offer many advantages including:
• No moving parts
• Small and lightweight
• Maintenance-free
• Acoustically silent and electrically "quiet"
• Heating and cooling with the same module (including temperature
cycling)
• Wide operating temperature range
• Highly precise temperature control (to within 0.1°C)
• Operation in any orientation, zero gravity and high G- levels
• Environmentally friendly
• Sub-ambient cooling
• Cooling to very low temperatures (-80°C)
Heat Spreader:
A heat spreader is most often simply a copper plate, having high thermal
conductivity. Functionally, it is a primary heat exchanger that moves heat
between a heat source and a secondary heat exchanger. The secondary
heat exchanger is always larger in cross sectional area, surface area and
volume. By definition, the heat is "spread out", such that the secondary heat
exchanger has a larger cross sectional area contacting the heat spreader

than the heat source. The heat flow is the same in both heat exchangers,
but the heat flux density is less in the secondary, so it can be made of a less
expensive material such as aluminium, and is a better match to an air heat
exchanger, since the low heat transfer coefficient for air convection is
adequate for a low heat flux. A heat spreader is generally used if and only if
the heat source tends to have a high heat-flux density, (high heat flow per
unit area), and for whatever reason, heat cannot be conducted away
effectively by the secondary heat exchanger. For instance, this may be
because it is air-cooled, giving it a lower heat transfer coefficient than if it
were liquid-cooled. A high enough heat exchanger transfer coefficient is
often sufficient to avoid the need for a heat spreader.
Back Pack Helmet:
Brief Description of Back-pack System:
System with Hybrid heat Sink (Convective and Heat Pipe both) Unit with TE
Module, Hybrid heat Sink, Small fans, Blower and Rechargeable Battery to
be carried as back pack. Electrical switch is fixed with waist belt. Cold Air, by
blower through flexible pipe, to be carried to head. The flexible pipe will
enter through holes in the helmet and through the small holes in the pipe,
the cold air will be distributed to cool the head. The system will be supplied
with a charger to recharge the discharged battery. Additional head load is
almost nil.
3D Modelling & Numerical Simulation of Heat transfer of
Thermoelectric Cooling / Heating Helmet for Back-pack system:
This simulation analysis deals with the performance prediction and
analysis of a thermoelectric cooling system through Computational Fluid

Dynamics (CFD). The system is divided into two parts namely cold side
and hot side of thermoelectric module (TEC). The cold side components
are blower, divergent duct, cold plate and convergent duct whereas the
hot side components are aluminium block, heat pipe, fins and fan. Two
different designs for cold plate are considered. The cold side analysis is
performed through CFD and hot side calculation is based on empirical
formulas. A comparative study of performance of these designs is carried
out for different types of blower and hot side fan combinations for three
different TEC modules.
Introduction
Application Objective
Working temperatures in the premises near hot furnaces are usually higher;
hence working under this high temperature condition is very uncomfortable.
A helmet design is proposed which will cool the high temperature air to
comfortable level and supplies it to head of the miner. This purpose is
fulfilled by use of many components like blower, cold plate assembly,
thermoelectric modules, fins and fans etc. Two designs of cold plate are
proposed with analysis parameters based on different blowers, fans and TEC
modules. Figure 1-1 shows the purpose of this analysis.
Components and Data
Heat Sink Assembly
Figure 1 shows overall assembly of hybrid system. It has components like
inlet duct (divergent), cold plate, outlet duct (convergent), TEC modules,
Aluminium block, heat pipe, fins, and fan.

Blower
Hot ambient air is forced to flow through cold plate by blower. Proposed
blower fans are DC Radial fans of capacity 17 CFM and capacity 35 CFM. The
type of blower used is shown in Figure 2.
Cold Plate
Divergent and convergent ducts guide the flow through cold plate. Cold
plate has several baffles which retards the flow and helps in heat transfer.
Two cold plate designs are considered for analysis.
TEC modules
TEC module takes heat from hot air through the cold side and dissipates it
through hot side. It has operating requirement of 12 V and 4.14 Amp. Input
power is 15 W. Two TEC modules will be used in parallel on one side of cold
plate. Another two types of TEC modules are also proposed.
Heat Pipe
Heat dissipated from hot side of TEC modules is transferred to heat pipe
through Aluminium block. It is assumed that there is no heat loss
(convection and conduction) from Aluminium block and hot side TEC
temperature is taken as input to heat pipe.

Hot Side Fins
A number of equally spaced fins are used to finally dissipate heat to the
atmosphere. Heat transfer rate depends on thickness of fins and spacing
between them.
Hot Side Fan
It is used to facilitate forced convection heat transfer from hot side fins.
Volume flow rate for fan is being investigated for desired design
performance. The type of fan used in this analysis is shown in Figure 3.
Methodology
CFD Analysis
Flow through divergent duct, cold plate, and convergent duct is analyzed
using CFD tools. Domain as shown in Figure 4 is divided into several
hexahedral cells and with proper boundary conditions, each property is
computed at the centers of cells by solving standard fluid flow equations at
each time step. Outcome of CFD simulation is in terms of flow and
temperature profiles.
System Level Analysis
Outcome of initial CFD analysis is taken as base for predicting performance
of the system. Volume flow rate value for blower is chosen first to match
cold plate pressure drop from given blower characteristics. For hot side fan,
similar procedure is performed. Then corresponding cold plate outlet
temperature is calculated which matches TEC module input power

requirements. From CFD analysis, mean baffle temperature is predicted and
corresponding heat transfer coefficient value is calculated. This calculated
heat transfer coefficient is used to predict new coefficient for different
values of blower and hot side fan volume flow rate. Finally values of
temperature at the hot side of TEC module and total heat dissipated from
hot side are then calculated.
The details of the system level analysis are provided at the end of this
document.
Initial Design
Design Description
Drawings and dimensions
Figure 4 shows dimensions of Initial cold plate.
CFD Simulation
Domain Creation and Mesh Generation
First, extents of the domain are defined and a 3D structured multi-block
topology is created. Several planes are cut at boundaries of cold plate
baffles. Thickness of cold plate and each baffle are assigned label "solid".
Rest of the domain is "fluid". Generated mesh is converted into hexahedral
unstructured mesh which is given as input to the CFD solver.
Figure 5 shows multi-block topology used for this simulation. The figures 6
and 7 show corresponding "fluid" and "solid" zones in generated volume
mesh respectively.

Boundary Conditions
Figure 8 shows boundary conditions applied to the CFD domain in order to
carry out the simulations. The following data shows type of boundary
conditions and values specified:
Inflow Boundary Condition
V = 0.67587 m/s
T = 50°C
Outflow Boundary Condition
T = 27°C
Outflow Boundary Condition
For each TEC module, Heat flux value is calculated as,
q = 15 W / (0.03 x 0.03) m2 = 16666.67 W/m2
All other boundaries are considered as adiabatic walls.
Flow Property Variation
Figures 9 and 10 show Pressure and Velocity-X variations computed through
CFD for the domain.
Temperature Distribution
Figure 11 shows temperature profile (in °K) on the section taken at the
middle of domain when viewed from front. Figure 12 shows the temperature
contours when section is taken along normal plane when viewed from top.

CFD Simulation Outputs
With given flow and thermal boundary conditions, the computed pressure
drop across the cold plate is 77.5 Pa for which pressure coefficient is
calculated. The temperature values at cold side of TEC modules are
22.886°C and 22.284°C for TEC1 and TEC2 respectively. Heat transfer
coefficient obtained is 55.58 W/m2 and mean cold plate baffle temperature is
26.601°C.
Performance Estimation
For requirement of given TEC capacity and given cold plate inlet and outlet
temperature values, blower volume flow rate is 2.24265 CFM and divergent
inlet velocity is 0.6759 m/s. TEC cold side and hot side temperature values
are 24.828°C and 56°C respectively. The input power requirement to TEC
modules is 49.76 W. Corresponding total heat dissipated from hot side is
79.76 W.
This design is corresponding to theoretically calculated blower CFM value. If
a blower with this CFM value is available, it can be directly used. Otherwise
a corresponding point on blower characteristics (for available given blower)
is chosen.
Design Analysis
Component Selected
Table 1-1 shows three different types of TEC modules considered in this
analysis. Table 1-2 shows volume flow rate for various hot side fan types
used.

Cold and Hot Side Calculations for different TEC Modules
The tables 1-4, 1-5 and 1-6 show cold and hot side analysis of Alternate
Design for TEC1, TEC2 and TEC3 respectively with blower.

Table 1-6 Initial Design - Calculations for Blower (TEC3)
Figure 14 shows variation of cold plate exit temperature with hot side fan
with different TEC modules.
Calculations for Blower - Blower Characteristics
Table 1-7 shows values for the points chosen on blower characteristics curve as
given in Fig 21.


Table 1-7 Initial Design - Blower Characteristics
Cold and Hot Side Calculations for different TEC Modules
The tables 1-8, 1-9 and 1-10 show cold and hot side analysis of Alternate
Design for TEC1, TEC2 and TEC3 respectively with blower.

Figure 16 shows variation of cold plate exit temperature with hot side fan
with different TEC modules.
Alternate Design
Design Description
Drawings and dimensions
Figure 17 shows design of Alternate cold plate. The corresponding
dimensions are also given in the figure.
CFD Simulation
Volume Mesh
Figure 18 shows CFD domain volume mesh generated for Alternate design.
Boundary Conditions
Figure 19 shows boundary conditions applied to the CFD domain in order to
carry out the simulations. The following data shows type of boundary
conditions and values specified:
Inflow Boundary Condition
V = 0.67587 m/s
T = 50°C
Outflow Boundary Condition
T = 27°C
Outflow Boundary Condition
For each TEC module, Heat flux value is calculated as,

q = 15 W / (0.03 x 0.034) m2 = 14705.88 W/m2
All other boundaries are considered as adiabatic walls.
Flow Property Variations
Figures 20 and 21 show Pressure and Velocity-X variations computed
through CFD for the domain.
Temperature Distribution
Figure 22 shows temperature profile on the section taken at the middle of
domain when viewed from front for alternate design. Figure 24 shows the
temperature contours when section is taken along normal plane when
viewed from side for alternate design.
CFD Simulation Outputs
The pressure drop across alternate design cold plate calculated is 172.55 Pa
and pressure coefficient is calculated from this value. Temperatures on the
cold side of TEC modules are obtained as 26.95°C and 23.95°C respectively.
Heat transfer coefficient is 39.9 W/m2-K and baffle mean temperature is
27.94°C.
Performance Estimation
For requirement of given TEC capacity and given cold plate inlet and outlet
temperature values, blower volume flow rate is 2.24265 CFM and divergent
inlet velocity is 0.6759 m/s. TEC cold side and hot side temperature values
are 25.433°C and 56.605°C respectively. The input power requirement to

TEC modules is 49.68 W. Corresponding total heat dissipated from hot side
is 79.73 W.
This design is corresponding to theoretically calculated blower CFM value. If
a blower with this CFM value is available, it can be directly used. Otherwise
a corresponding point on blower characteristics (for available given blower)
is chosen.
Design Analysis
Component Selected
Table 2-1 shows three different types of TEC modules considered in this
analysis. The Tables 2-2 and 2-3 show blower and hot side fan types used.


Calculations for Blower
Blower Characteristics
Table 2-3 shows values for the points chosen on blower characteristics curve
as given in Figure 24.

Table 2-3 Alternate Design - Blower Characteristics
Cold and Hot Side Calculations for different TEC Modules
The tables 2-4, 2-5 and 2-6 show cold and hot side analysis of Alternate
Design for TEC1, TEC2 and TEC3 respectively with blower.



Table 2-6 Alternate Design - Calculations for Blower (TEC3)
Figure 25 shows variation of cold plate exit temperature with hot side fan
with different TEC modules for alternate design.
Calculations for Blower
Blower Characteristics
Table 2-8 shows values for the points chosen on blower characteristics curve
as given in Figure 27.

Table 2-7 Alternate Design - Blower Characteristics
Cold and Hot Side Calculations for different TEC Modules
The tables 2-8, 2-9 and 2-10 show cold and hot side analysis of Alternate
Design for TEC1, TEC2 and TEC3 respectively with blower.


Figure 28 shows the complete back-packed air circulating helmet system.
Conclusion
A systematic comparative study on performance of thermoelectric cooling
system is performed. The flow and thermal patterns are predicted through
CFD for initial calculations for both cold plate designs. These calculations are
then used to predict hot side system performance for different combinations
of blowers, hot side fans and thermoelectric modules.

By using the combination of components available for design, it is possible
to get the cooling system to perform to obtain either very low exit
temperature or very high flow rate.
The above conclusion is visible from the following graph, where relationship
between volume flow rate & cold plate exit temperature is evident.
Cold Side Components
Blower:
Even though the maximum rated capacity for chosen fan is high (17 CFM &
35 CFM), when integrated with cold plate designs, these generate relatively
low volume flow rates i.e. between 1.5 & 5.0 CFM. This is because of large
pressure drop that cold plates cause to flow.
However, the blower options are appropriate as they are designed to
generate enough pressure rise to make flow through the cold plate. The
final selection is based on minimum flow rate requirements
Cold Plate:
The alternate design is found to generate 2.2 times more pressure drop
compared to initial design. On a pure heat transfer basis, initial cold plate
design is 40% more efficient compared to alternate design.
Accordingly, use of initial cold plate design is recommended.

TEC Module:
Keeping all components same, choosing Type 3 TEC module brings down
the exit temperature by 2.3°C compared to what is obtained by using Type 1
TEC.
Similarly, choosing Type 2 TEC module brings down the exit temperature by
4.2°C compared to what is obtained by using Type 1 TEC.
Hot Side Components
The heat loss rate through the hot side can be written as:

The effectiveness of hot side design can be gauged from the value of
effective heat transfer coefficient, hetr. Higher the value of heff, more would
be the amount of heat that can be dissipated away. Typical value of hetf is
25.8 W/°C. Therefore, a 3 degree difference between TEC hot side (53°C) &
ambient temperature (50°C) would dissipate 75W of heat.
As a general guideline, analysis of data reveals that 10% increase in hefr
reduces the cold plate exit temperature by 0.13°C.
Hot Side Fans:
The study has investigated usage of four axial fans with volume flow rates of
88.30, 108.89, 120.66 & 147.15 CFM, respectively. On analysis, these four
fans give value of heff as 22.6W/°C, 25.1W/°C, 26.4W/°C & 29.2W/°C
respectively.

For every 20CFM increase in hot side fan volume flow rate, we find a
marginal drop of 0.11°C in cold plate exit temperature. However, reducing
CFM values can have drastic deterioration in heff because width of thermal
boundary layers on fin would increase & may overlap.
Hot Side Fins:
The heat transfer coefficient is directly proportional to the extended area
generated by the fins. Hence, by increasing number of fins, a proportional
increase in heat transfer coefficient can be obtained. However, it is not
possible to increase the number of fins on the available heat pipe because
(i) it is difficult to manufacture fin assembly with gap less than 2 mm & (ii)
lesser gap would lead to deterioration in heff because width of thermal
boundary layers on fin would increase & may overlap.
Heat Pipe:
Increasing the length of heat pipe & keeping the number of fins same would
not have any impact on heat dissipation quality of hot side assembly.
However, introduction of more number of fins on the heat pipe (keeping the
gap between fins as 2 mm), improvement in heat dissipation is possible.
Analysis of numerical data reveals that for obtaining a reduction of 0.5°C in
cold side exit temperature, the heat pipe length should be increased from
150 mm to 200 mm.
*Typical photographs of Back-pack system are shown in Figure 28.
*Typical experimental results of the Back-pack system are shown in
Annexure (A) to Annexure (C).

SYTEM LEVEL CALCULATIONS
Heat transfer rate,